1. Field
The present invention relates generally to optical systems for correcting the variation of an area exposed by a light beam due to the obliquity of the light beam, and more specifically this invention relates to optical systems, e.g., holographic data storage systems, that decrease the width of a light beam as the obliquity of the light beam increases.
2. Related Art
Many optical systems and applications vary the angle of incidence of light beams with respect to a plane or surface and vary the width of the light beams to maintain a substantially uniform area or footprint of the light beam with respect to the plane or surface. For instance, various optical systems and applications such as holographic data storage systems, laser processing systems, material property measurement systems, and the like, desirably maintain a substantially uniform footprint area over various angles of incidence.
Holographic data storage systems, for example, store information or data based on the concept of a signal beam interfering with a reference beam at various angles with respect to a holographic storage medium. The interference of the signal beam and the reference beam creates a holographic representation, i.e., a hologram, of data elements as a pattern of varying refractive index and/or absorption imprinted in a volume of a storage or recording medium such as a photopolymer or photorefractive crystal. Combining a data-encoded signal beam, referred to as an object beam, with a reference beam can create the interference pattern at the storage medium. A spatial light modulator (SLM) or lithographic data mask, for example, may create the data-encoded signal beam. The interference pattern induces material alterations in the storage medium that generate the hologram. The formation of the hologram in the storage medium is generally a function of the relative amplitudes and polarization states of, and phase differences between, the signal beam and the reference beam. The hologram is also dependent on the wavelengths and angles at which the signal beam and the reference beam are projected into the storage medium. After a hologram is created in the storage medium, projecting the reference beam into the storage medium interacts and reconstructs the original data-encoded signal beam. The reconstructed signal beam may be detected with a detector, such as a CMOS photo-detector array or the like. The recovered data may then be decoded by the photo-detector array into the original encoded data.
A holographic storage medium may include a variety of materials and take a variety of forms. For example, it may include a film containing dispersed silver halide particles, photosensitive polymer films (“photopolymers”) or a freestanding crystal such as iron-doped LiNbO3 crystal. U.S. Pat. No. 6,103,454, entitled RECORDING MEDIUM AND PROCESS FOR FORMING MEDIUM, generally describes several types of photopolymers suitable for use in holographic storage media.
FIG. 1 illustrates the basic components of an exemplary holographic storage system 10. System 10 contains an SLM 12, a holographic storage medium 14, and a sensor 16. SLM 12 encodes beam 20 with an object image. The image is stored by interfering the encoded data beam 20 with a reference beam 22 at a location on or within holographic storage medium 14. The interference creates an interference pattern (or hologram) that is captured within medium 14 as a pattern of, for example, a holographic refractive index grating.
To increase the storage capacity of holographic storage media, more than one holographic image may be stored at a single location, or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 22, depending on the particular reference beam employed. Data beam 20 typically passes through lenses 30 before being intersected with reference beam 22 in the medium 14. It is possible for reference beam 22 to pass through lenses 32 before this intersection. Once data is stored in medium 14, it is possible to retrieve the data by intersecting a reference beam 22 with medium 14 at the same location and at the same angle, wavelength, or phase at which a reference beam 22 was directed during storage of the data. The reconstructed data beam passes through one or more lenses 34 and is detected by sensor 16. Sensor 16, is for example, a charged coupled device or an active pixel sensor.
Multiple images or data pages of information may be stored in the same volume by varying the angle of the reference beam 22 during recording. This process is generally referred to as angle multiplexing, where successive images are recorded in the same volume using varying reference beam 22 angles. Varying the angle of reference beam 22 over a wide range of angles with respect to the volume increases the number of images or data to be stored in the volume.
Varying the angle of the reference beam 22, however, increases the area of the holographic storage medium 14 exposed by reference beam 22. The area of holographic storage medium 14 exposed by reference beam 22 depends upon the angle of incidence of reference beam 22 with respect to the surface of holographic storage medium 14 (“the obliquity”), and the width of reference beam 22. The area exposed by reference beam 22 is related to the capacity of the holographic storage medium 14 to store data; generally, a larger area exposed by reference beam 22 results in a reduction in the capacity of the holographic storage medium 14 per unit volume. Accordingly, it is generally desired to maintain a substantially uniform area as the angle of incidence varies.
Obliquity has been corrected in optical systems, including holographic data storage systems, with the use of a complex set of stationary prisms and optical elements (see, e.g., Coufal et al., “Tamarack Optical Head Holographic Storage” in Holographic Data Storage, 343–357 (2000)). FIG. 2 shows an exemplary obliquity correction system using two prisms 226 and 228 and three lens components 230, 232, and 234. In FIG. 2, light beams 224 are reflected off of scanning mirror 222 to first prism 226. The light beams exiting first prism 226 proceed to second prism 228 and through lens components 230, 232, and 234 to a surface 236. The width of the reference beam decreases as the angle of the reference beam with respect to the normal of surface 236 increases.